Production of halogens such as chlorine through the electrolysis of a sodium chloride solution with caustic (NaOH) as a co-product is a great industry. The Chlor-Alkali industry produces millions of tons of chlorine and caustic soda per year. The principal electrolytic processes by which chlorine has been produced are the so-called mercury cell and diaphragm cell processes. The mercury cell process involves the electrolysis of an alkaline metal chloride solution in a cell between a graphite or metal anode (DSA - Dimensionally Stable Anode). Chlorine is liberated at the anode and the alkali metal is deposited into the mercury in the form of an alkali metal amalgam. The latter is treated in a decomposition reaction in which the amalgam is reacted with water to form caustic soda and hydrogen. However, the mercury cell process for generation of chlorine is, for all practical purposes, now obsolete. Mercury is such a hazardous substance and governmental regulatory provisions for the control of mercury and other types of pollution are becoming so stringent that the days of the mercury cell are over. However, beyond the pollution aspect and the environmental problems associated with the use of mercury cells for chlorine generation, mercury cells are complex and expensive. The use of mercury itself introduces problems relative to the size and complexity of the cell because of the care needed in handling the material. Mercury is expensive and substantial quantities must be used. Not the least of the economic problems with the process is the need for a decomposition step, and the attendant equipment, to produce the caustic soda and hydrogen.
The diaphragm cell on the other hand does not involve the use of mercury, but contains foraminous electrodes separated by a microporous diaphragm. The space between the electrodes is filled with a brine solution and separated by a microporous diaphragm which may take the form of an overlying porous diaphragm which separates the catholyte and anolyte compartments. One of the serious disadvantages of a diaphragm cell is the fact that pores in the diaphragm permit mass transfer or hydraulic flow of sodium chloride solutions across the diaphragm. As a result, the catholyte, i.e., the caustic produced at the cathode contains substantial amounts of sodium chloride. This results in the production of an impure and dilute caustic. On the other hand, hydroxide produced at the cathode can back migrate through the porous separator to the anode where it is electrolyzed producing oxygen. Production of oxygen at the anode is very undesirable for several reasons. Production of oxygen at the anode not only results in low purity chlorine, but also oxygen attacks the anode electrode.
Because the mass transfer of the anolyte and catholyte between the chambers produces so many undesirable effects, a number of arrangements have been proposed to minimize or eliminate these problems - one of these is maintaining a pressure differential across the diaphragm to ensure that the mass transfer of the electrolytes between the anolyte and catholyte chambers is minimized. However, such solutions are at best only partially effective.
In order to overcome the disadvantages associated with the diaphragm cell and the mass transfer of electrolyte across the porous diaphragm, it has been suggested that ionically permselective membranes be utilized in chlorine generating cells to separate the anolyte and catholyte chambers. The permselective membranes used in these cells are typically cationic membranes in that they permit the selective passage of positive cations while minimizing passage of negatively charged anions. Since these membranes are not porous, they do have a tendency to inhibit the back migration of the caustic from the catholyte chamber to the anolyte chamber and similarly to prevent the brine anolyte from being transported to the catholyte chamber and diluting the caustic. It has been found, however, that membrane cells are still subject to certain shortcomings which limit their widespread use. One of the principal shortcomings of the membrane type cell as they are known to date is that they were characterized by high cell voltage. This is only in part due to the membrane characteristic itself. It was in great part due to the fact that the known membrane cell construction utilize electrodes which are physically spaced from the membrane. As a result of the physical spacing between the electrodes and the membrane, the cell, in addition to the IR drop across the membrane, involves electrolyte IR drops in the electrolyte between the electrodes and the membrane prior to ion transport and are also subject to voltage drops due to gas bubble formation or mass transfer effects. That is, since the catalytic electrodes are spaced from the membrane, the chlorine is generated away from the membrane. This results in the formation of a gaseous layer between the electrode and the membrane. This gaseous layer interrupts the electrolyte path between the electrode and the membrane, thereby partially blocking the ions from the membrane. This interruption of the electrolyte path between the electrode and membrane, of course, introduces an additional IR drop which increases the cell voltage required for generation of the chlorine and obviously reduces the voltage efficiency of the cell.